This invention relates to transverse magnetic printing heads and methods for economically manufacturing such structures. More particularly, this invention relates to a novel printed circuit and magnetic comb structure for the digit and word drive lines in a coincident current, transverse magnetic printing head. A significant advantage gained by the structure produced herein is that the circuit provided at the matrix printing head is planar and, therefore, can be readily fabricated upon a flexible printed circuit substrate with high print resolution.
A detailed description of magnetic printing may be found in U.S. Pat. No. 4,097,871, issued to Berkowitz et al., the same inventors as herein, and which is assigned to the same assignee as this application. It is to be particularly noted, however that, in a transverse magnetic printing head, the direction of orientation of the magnetic field impressed on the recording medium is perpendicular to the direction of movement of the medium.
Magnetic printing basically involves the transfer of a dry magnetic ink image from a ferromagnetic recording medium to a permanent paper medium. To produce the magnetic image on the recording medium which attracts the magnetic ink from a reservoir to the magnetic recording medium in a transverse magnetic printing system, a long comb-like structure is provided which is oriented in a direction perpendicular to the direction of travel of the magnetic recording medium. The comb itself is composed of a material of high magnetic permeability, thereby facilitating the concentration of lines of magnetic flux at the tips of the teeth of the comb. Typically, a plurality of current carrying conductors is threaded through each of the gaps in the comb structure formed by its teeth and the current in these conductors controls the level of magnetic flux at the tips of the teeth of the comb. These regions of increased magnetic flux being in a position near to the magnetic recording medium permit the selected magnetization of regions in the medium which regions thereby attract the magnetic ink. In this manner, the currents in the conductors threaded through the teeth of the comb control the patterning of the magnetic ink on the ferromagnetic recording medium. The ink pattern is later transferred to a paper medium to which the pattern is permanently fixed and the pattern on the magnetic recording medium is subsequently magnetically erased prior to the recording of new information thereon and prior to the subsequent contact of that region of the medium with the paper.
To achieve high resolution, the teeth of the comb must be spaced relatively close together. For example, in recent magnetic printing head structures, the teeth of the comb are spaced to form approximately 120 gaps per inch providing for only a 4 mil spacing between adjacent teeth. Additionally, experiments indicate that 8 to 10 ampere-turns through the conductors is required to provide a magnetic image strong enough to cause a transfer of magnetic ink to the magnetic recording medium. In some prior art conductive windings through the teeth of the comb have had to pass through the gap two or three times to achieve the necessary ampere-turns necessary to provide adequate magnetic ink transfer, and this is difficult with the small spacing involved between comb teeth. For example, if a matrix approach (to be more particularly described below) is used, both a "word" drive and a "digit" drive line must be provided for each gap. If a 14 inch paper width is used, which is not uncommon, then at a resolution of 120 gaps per inch, there is required a total of 1680 gaps. In the past, each of these gaps have had to be threaded with from four to six word line and digit drive line conductors. Fabrication of such a printing head therefore required a great number of separate operations each requiring the placement of a single conductor within a slot which is typically only four mils wide. Even if such a process could be automated, the requisite equipment would be extremely expensive.